[iOS] Fix empty frameID DCHECK when FrameID is disabled

Bug: none
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Id18a4a4fb0495a77dfae370f8e5cf2118614aae9
Reviewed-on: https://chromium-review.googlesource.com/1249207Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595011}

ios/web/web_state/web_frame_util.mm

@@ -29,6 +29,8 @@ std::string GetMainWebFrameId(WebState* web_state) {
 }
 
 WebFrame* GetWebFrameWithId(WebState* web_state, const std::string& frame_id) {
+  if (frame_id.size() == 0)
+    return nullptr;
   WebFramesManager* manager = WebFramesManager::FromWebState(web_state);
   DCHECK(manager);
   return manager->GetFrameWithId(frame_id);